The field of this invention is the detection of minute concentrations of formaldehyde vapor in gases, particularly the detection of formaldehyde concentrations in air in the ppm and sub-ppm range. The kind of detection involved is a colorimetric reaction producing a chromogen the color intensity of which is proportional to the amount of formaldehyde vapor to which the detector is exposed. Such detectors can be qualitative, semi-quantitative, or quantitative, and can be read with standard reflectance spectrophotometers, or visually by the eye using color comparison standards. The invention here relates to the novel chemistry of the detector, and its method of preparation and use.
No directly relevant prior art is known. Prior methods for detecting low concentrations of formaldehyde in air have followed entirely different principles and used entirely different chemistry. However, the primary reagent used to prepare the detector of this invention is a known colorimetric reagent. This reagent is 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole (AHMT). This compound is available commercially under the trademark name "Purpald" from Aldrich Chemical Co., Milwaukee, Wis. Heretofore, it has been used primarily to determine aldehydes by colorimetric reaction in alkaline aqueous solution. See Chromatographia, 5, 307308 (1972) and Chemical Communications, 1970, 1719-1720. AHMT combines with phase-transfer catalysts so as to provide an aldehyde classification test. J. Chem. Educ., 55, 206 (1978). It has also been shown that the compound AHMT is the triazole formed as the common product in the hydrazinolysis of many thioureido compounds. Anal. Chem., 41, 1324-1327 (1969).